The Frontal Cortex

Three Mysteries of Evolution

I’ve really enjoyed Olivia Judson’s columns on Times $elect. They’ve been funny, eloquent and haven’t shied away from the biological nitty-gritty. In her last column, she ends with a meditation on three questions she wants evolutionary biologists to solve:

The first is metamorphosis. Everyone knows a caterpillar becomes a butterfly; and it is easy to work out why it might be useful to split your life between growing (caterpillar) and loving (butterfly). But what I find peculiar is the manner of becoming a butterfly: within the pupa, the caterpillar breaks down its body, including much of its brain, and reconstitutes itself along different lines. It’s a kind of reincarnation. I’d love to know how this evolved. (This may already be known to someone; if it is, I have so far failed to discover to whom.) Even weirder are some of the transformations that take place in the sea. Here, animals such as sea urchins actually change the symmetry of their bodies when they metamorphose — they go from bilateral symmetry, like a human face, to radial symmetry, like a wheel. Weirder still, rather than recycle their old bodies, as caterpillars do, many of the marine critters throw large chunks of their old bodies away. The evolution of such radical transformations seems hard to imagine — and yet, it’s a phenomenon that has arisen many times, so it can’t be that hard to evolve.

The second is meiosis. This is the kind of cell division by which sperm and eggs are made — so it’s essential for sex as we know it. Meiosis is a strange — and horrendously involved — procedure. Why strange? Well, each of us has two sets of chromosomes — one from our mother, and one from our father. When we make eggs or sperm, we divide up our two sets so that each egg or sperm has one complete set. You might think that to do this, you’d just divide the precursor cell in two, with each descendant having one set of chromosomes. But no. You double the chromosomes — four sets — then split the cell into two, then two again. Odd. But it’s how meiosis could have evolved that really makes my mind boggle. Here, working out the early steps is complicated by the fact that meiosis seems to have evolved only once — most organisms seem to be doing minor variations on the same major theme — so we can’t compare and contrast. Worse, the origins of meiosis are shrouded in the mists of deep time.

Speaking of deep time brings me to my third puzzling phenomenon: the origin of life. This, of course, is the biggest mystery in all biology. Did life arise here, or arrive here? If it arose here, how? We can at least imagine the basic steps. One of the first is the invention of membranes — for living cells are separate compartments, and membranes are the containers. Or, if life arrived here, where did it come from — and where did it start? Are other planets home to life — and if so, what are they like?

Comments

  1. #1 Curtis
    June 30, 2006

    I have two more things that seem to be open puzzles:
    1) This one is fairly obvious – consciousness in humans. Why are we not simply machines, but also have qualia? I know this has received some philosophical discussion, but I’m curious if anyone has read anything interesting about the issue.

    2) The other is what might be called (in my use of economics terms) network externalities. To see what I mean by this term, one common example of a ‘network externality’ in economics was the introduction of the gold Sacagawea dollar coin a couple of years ago: for consumers to have the incentive to use it, enough retailers, businesses, vending machines, etc. would have to have the means and willingness to accept them. But, for the suppliers to provide the means for accepting those dollars (in the form of new cash registers & coin receptacles), there needed to be a substantial amount of consumers to provide them the incentive to do so. So how might this apply to evolution? One obvious way is communication: if a herd animal makes a call to its fellow herd members that a predator is near, the other herd animals must be capable of responding and recognizing the call. The question is: how was it adaptive for the first animal of that species to communicate, without other animals of the species able to respond or process the act of communication?

    It seems simple for how communication could continue to evolve once introduced, and explanations can be made with game theory. But, I’m puzzled how it could have originally been introduced into a species, and how an adaptive gene could have originally been introduced. One thought I had was that maybe the original act of communication was not really intended to be communication. In other words, the call was used for some other adaptive purpose, and eventually the receptive communication gene got introduced. Thus, communication could be a by-product of another adaptation.

    Does anybody have any thoughts about this, or know of any previous discussion to this topic – I’m sure it must have been discussed in the literature before.

  2. #2 Eric Irvine
    June 30, 2006

    Well, i’ll hazard a few insights (guesses):

    1) Metamorphosis: Thinking of this process as akin to the development of a chick embryo turning into a chicken it doesn’t seem so startling. Inside of the egg, a chicken has a supply of ample nourishment for it to complete its transformation; Fly maggots simply forage externally without outside help, and when they are sufficiently mature they can continue with their transformation into an adult fly. The process is simply an altered version of development: mammals use their placenta to nourish; reptiles their eggs; and for many insects and other species, the young must develop by finding their own food.

    In addition, it is startling to notice how similar embryo’s appearance stays constant across many species; As Darwin noted, this shows how natural selection can work on different stages of life – certainly food for thought.

    2) Meosis: we will, in all likelyhood, never know how it evolved; evolutionary data that old simply doesn’t exhist anymore.

    3) Life: After considering the size of the distances between planets, solar systems, and galaxies, I have little doubt that life sprung from the Earth. Why would life that so thrived on Earth’s environment spark somewhere else? It makes more sense that life was custom made to Earth, from Earth. I also doubt we will ever know how life really arose on Earth – the best we can do is recreate the process, which is not proving what actually happened.

    @Curtis

    1) Consciousness: conscious decision making allows for greater flexibility in dealing with the environment around an organism; therefore, those that are conscious are more likely to survive and reproduce.

    How did this come about? I have no clue. However, I will put the idea forward that the division between animals and man’s intellect is mostly arbitrary. All you have to do is go out and watch nature to see that it is not filled with automata, but with beings that are thinking and considering – albeit in a different manner from ourselves.

    In my opinion the very concept of consciousness is wrong and dangerous. So i’ll pull a socrates and ask, what is consciousness?

    2) Speaking first or comprehension?: I would guess hearing came first without comprehension. The environment is filled with sounds that might alert an animal to danger. Over time, creatures could begin to comprehend certain sounds as alerts. Later, animals could produce the alarms themselves. Eventually, this could lead to speaking.

    It’s all from Darwin: little steps were taken over time to reach an ultimate conclusion that we see today – nothing happened overnight.

  3. #3 Curtis
    July 1, 2006

    Maybe I should have been more specific about the consciousness thing: I wasn’t referring to the type of conscious thoughts/activities that you are. Instead, I was referring to qualia (see: http://plato.stanford.edu/entries/qualia/ and for the difference types of things referred to by ‘consciousness': http://plato.stanford.edu/entries/consciousness/#2.2). I could see that an argument could be formed that would posit that the type of consciousness you were talking about necessarily needs qualia, but I could also see the reverse argument. If the latter argument turned out to be correct, it would imply that no evolutionary advantage would accompany qualia. While I do have an interest in philosophy of mind, all of the issues are just so overwhelmingly confusing that I really don’t have much to add to discussion of it.

  4. #4 fussball
    March 11, 2009

    Gute Arbeit hier! Gute Inhalte.

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